Electrical power tools, such as variable speed drills and power screwdrivers, typically include a motor control circuit that is adapted to control the speed of the output spindle of the tool by controlling the amount of current supplied to the motor. The desired motor speed is usually selected by the operator by varying the position of the trigger switch.
If the power tool is provided with an open loop motor control circuit, the speed of the output spindle of the tool will decrease as the tool is loaded and the current drawn by the motor will increase. Accordingly, if a relatively constant output speed is desired, the operator must manually compensate for the reduction in motor speed as the tool is loaded by further retracting the trigger switch to increase the power applied to the motor. If the power tool is provided with a closed loop motor control circuit, the control circuit is typically designed to automatically increase the amount of power supplied to the motor as the output spindle of the tool is loaded in order to maintain the desired speed.
Thus, when employed in a power screwdriver to drive a screw into a workpiece, for example, the current drawn by the motor will increase as the torque required to drive the screw increases, regardless of whether the control circuit provides open or closed loop control. This operation will continue until either the operator releases the trigger or the motor stalls as the increased torque required to drive the screw exceeds the torque capacity of the tool. Consequently, the effectiveness of many portable power tools, particularly power screw-drivers, is directly related to the tool's maximum torque output capacity. Obviously, the greater the output capacity of the tool, the more useful and versatile the tool. However, in order to significantly increase the torque output capacity of a tool, it is generally regarded as being necessary (assuming changes to the gear train are not an option) to increase the size of the motor and, consequently, the size, weight, and cost of the tool.
Accordingly, it is the primary object of the present invention to provide a portable electrical power tool having a motor speed control circuit that is able to substantially increase the effective torque output of a power tool for a given size motor and gear train.
In addition, it is an object of the present invention to provide a portable electric power tool, such as a power screwdriver having a motor control circuit, that enables the operator to better control the torque output of the tool, which is particularly beneficial when driving a screw into a workpiece.
The motor control circuit employed in the present invention is able to achieve these objectives by intermittently pulsing the motor fully on and then fully off for predetermined periods of time after a threshold current level is attained. More specifically, it has been found that if the motor of a power drill is turned completely off for a length of time sufficient to allow the gear train coupled to the motor to at least partially "relax", and then full voltage is reapplied to the motor, the motor is able to build up potential energy before the looseness (i.e., backlash) is removed in the gear train. In effect, the motor is afforded a "running start" while the gear train is relaxed. When the backlash in the gear train is removed, the sudden impact of the motor torque on the gear train causes a sudden and high burst of torque to be imparted to the output spindle of the drill, and hence to the driving bit secured thereto. When this "full on" and "full off" control of the motor is alternately repeated, the motor is able to provide a series of bursts of torque to the gear train which in turn can be used to better finish driving a wood screw into and below the surface of a workpiece. The present control scheme thus provides better user control due to the fact that the screw does not rotate too much when static friction is overcome. Rather, with each torque pulse, static friction is overcome and the screw is incremented a fraction of a turn.
While the alternately full-on and full-off operation described above has been found to be particularly helpful and effective when used to drive wood screws and other like implements into a work surface, it has also been found to be an effective means for "breaking loose" a screw or like fastener which is tightly seated in a workpiece, where other forms of power tools such as conventional variable speed drills are unable to do so. By reversing the action of the variable speed drill and applying the alternating full-on and full-off operation described above, the bursts of high torque applied by the motor have been found to be extremely effective in overcoming the high level of stiction force required to initiate removal of such fasteners.
Accordingly, it is a further object of the present invention to provide an electrically driven power tool, such as a variable speed power drill, which incorporates a control circuit for controlling a motor thereof such that the motor can be alternately pulsed fully on and then fully off at a predetermined cycle time during operation of the drill.
It is another object of the present invention to provide an electrically driven power tool having such a control circuit that further provides an operator of the power tool with a means for adjusting the point at which the alternating full-on and full-off operation is initiated.
It is yet another object of the present invention to provide an electrically driven power tool which automatically enters the alternating full-on and full-off mode of operation when the current through the motor exceeds an operator adjustable threshold level setting.
Finally, it is an alternative object of the present invention to provide an electrically driven power tool that provides the operator with control over the magnitude of the torque bursts during the alternating phase of operation of the tool.